Components of gas turbine engines are exposed to very high temperature, high pressure combustion gasses containing moisture, oxygen and other corrosive compounds. Modern gas turbine engines may have firing temperatures that exceed 1,400° C., and temperatures ranging from approximately 1,500° C. to approximately 1,600° C. and higher are expected as the demand for even more efficient engines continues. Cobalt- and nickel-based superalloys are used to form many gas turbine components, but even these superalloy materials need to be aggressively cooled and/or insulated from the hot gas flow to survive long term operation in the combustion environment.
Ceramic matrix composite (CMC) materials have many potential applications in high temperature environments due to their ability to withstand and operate at temperatures in excess of those allowed for a non-insulated superalloy part. However, oxide and non-oxide CMCs can survive temperatures in excess of 1,200° C. for just limited time periods in a combustion environment. Furthermore, oxide-based CMCs cannot be cooled effectively with active cooling systems due to their low thermal conductivity and their limitations in cooling fluid path design due to manufacturing constraints. Non-oxide based CMCs can be aggressively cooled to withstand temperatures above 1200° C., but they are subject to environmental degradation that limits their useful life. To increase the operating temperature range and useful life for CMC materials, a high temperature insulation for a ceramic matrix composite material is described in U.S. Pat. No. 6,013,592.
Structural ceramic technology for gas turbine engines presently relies on silica-based materials. Silica-based non-oxides such as silicon carbide (SiC) and silicon nitride (Si3N4) are subject to both oxidation and attack by high temperature, high pressure water vapor. In this dual degradation mechanism, the silicon carbide or silicon nitride is oxidized to form a thermally grown oxide (SiO2) layer. This oxide layer then reacts with the high temperature, high pressure water vapor to form a volatile hydroxide species [Si(OH)x] which is then lost to the environment. Thus, surface recession occurs in a continual process as the protective SiO2 layer volatizes and the base ceramic oxidizes to replenish the lost SiO2. This process is enhanced by the high velocity gas stream in a gas turbine environment. Accordingly, improved materials are needed in advanced combustion turbine engines where firing temperatures may be in the range from approximately 1,500° C. to approximately 1,600° C. and higher.